The District of Columbia Department of the Environment recommends hiring a Certified Radon Tester who has completed a radon measurement certification course approved by NRPP and AARST.
InterNACHI’s free, online “Advanced Radon Measurement Service Provider Course” is NRPP- and AARST-approved and located is at http://www.nachi.org/radon-measurement-service-provider-online-course.htm
For details about home inspector and radon tester certifications in the District of Columbia, visit http://www.nachi.org/districtofcolumbia